Base station cooperation in a heterogeneous network is an important means to improve the performance, e.g., throughput, of communication systems. In the heterogeneous network, the transmission power from different base stations may be very different and accordingly the coverage areas of the base stations are different. Specifically, the coverage area of a high power base station, i.e. a high power node (macro) is much larger than the coverage area of a low power base station, i.e. a lower power node (LPN), and the coverage area of the LPN may overlap with that of the macro, as shown in FIG. 1. Because of the large coverage area of the macro and the small coverage area of the LPN, a large number of UEs are in the coverage area of the macro and will be served by the macro while only a small portion of UEs will be served by the LPNs, which results in a heavy load on the macro and an inefficient use of spectral resources of LPNs. To improve the spectral usage of LPNs as well as to reduce the load on the macro, it is preferred to assign more UEs to be served by the LPN, even when the power received from the macro is higher than the power received from the LPN. This is a so called traffic offloading.
There are multiple possible types of base station cooperation, and each type may yield a traffic offloading in different level. For example, as shown in FIG. 1, a joint transmission (JT) from the macro and the LPN1 or LPN2 causes more macro traffic than a JT from the LPN1 and LPN2, and hence is less preferred from the traffic offloading point of view. In other words, in case the macro is heavily loaded and the LPNs are lightly loaded, it is desired to avoid the joint transmission involving data transmitted from the macro, e.g., the joint transmission from macro and LPN1. In this document, joint transmission refers to that the UE receives data from multiple base stations simultaneously.
A simple operation that enables traffic offloading may be as following: first, each UE report one CQI to base stations, where the CQI is calculated at UE assuming a base station cooperation type friendly to traffic offloading; second, a scheduler included in the base station (which could alternatively be located separately from the base stations in the heterogeneous network) calculates Proportional Fairness (PF) metric of each UE based on the reported CQI to make a resource allocation; third, the scheduler allocates for example Physical Downlink Shared Channel (PDSCH) to UEs based on the calculated PF metric; note that here the PDSCH transmission in general may follow the base station cooperation assumed in UE CQI calculation.
To enable the more flexible traffic offloading, a further operation may be as following: first, each UE report two CQIs to base stations, where different CQI is calculated assuming different types of base station cooperation (and consequently assuming different traffic load); second, based on the load condition, a scheduler included in the base station (which could alternatively be located separately from the base stations in the heterogeneous network) internally decide on which subframe(s) the macro is allowed to carry out the first type and the second type of base station cooperation, respectively; third, the scheduler calculates Proportional Fairness (PF) metric of each UE on a certain subframe to make a resource allocation, the PF metric is calculated based on allowed base station cooperation on that certain subframe; fourth, the scheduler allocates for example PDSCH to UEs based on the calculated PF metric; note that here the PDSCH transmission in general may follow the base station cooperation assumed in the relevant UE CQI calculation.
Both of the above operations require the UE to report CQI(s). In general, different types of base station cooperation lead to different CQIs. Therefore, there is a problem how to decide which base station cooperation should be assumed in CQI calculation. In short, the problem is how to decide which CQI should be reported.
The first possible solution is to let the UE decide which CQI(s) should be reported. However the UE selected CQI could be very off because the traffic load conditions at base stations are unknown to the UE. If the UE merely selects the highest CQI(s), then it is likely that the highest CQI(s) cause heavy traffic load at the macro, which means that traffic offloading operation at base stations is not possible.
The second possible solution is to feedback CQIs based on a base station signalling. That is to say, the base station indicates UE which two CQIs should be reported. However, this solution has following defects. On one hand, the base station selection is not optimal because the base station does not know which CQI(s) are the highest at UE. Therefore base station selection does not guarantee optimal performance. On the other hand, even if in some case the base station has knowledge of which CQI(s) are the highest, the downlink signalling overhead is too much because highest CQI(s) may change dynamically. For example, if the signalling is send through Physical Downlink Control Channel (PDCCH), which is a dynamic downlink signalling, the downlink signalling overhead may be intolerable. If the signalling is send through Radio Resource Control (RRC), which is a semi-static downlink signalling, the configuration can not be changed quickly. Because the highest CQI may changes much faster than RRC configuration, the semi-statically configuration from base station maybe not yields best performance.